The present invention relates to stable frequency generation apparatus, and, more particularly, to a novel phase-locked loop circuit which is stabilized by a crystal oscillator.
It is known that information may be transmitted over power lines between a central facility and a multiplicity of remote locations, each tied to the power distribution network. Power line communications systems of this type are disclosed and claimed in U.S. Pat. Nos. 3,973,240, issued Aug. 3, 1976; 3,973,087, issued Aug. 3, 1976; 3,944,723, issued Mar. 16, 1976; 4,135,181, issued Jan. 16, 1979; and 4,161,720, issued July 17, 1979, all assigned to the assignee of the present application and all incorporated herein in their entirety by reference.
It is also known that the signal-to-noise ratio of the received signal can be greatly improved if the frequencies at which the signals are transmitted are odd multiples of the first sub-harmonic of the power line frequency (typically 60 Hertz (Hz.) in the United States). Improvements in interference suppression are described and claimed in U.S. Pat. Nos. 4,101,834, issued July 18, 1978, and 4,109,204, issued Aug. 22, 1978, both assigned to the assignee of the present application and incorporated herein in their entirety by reference. In power line communication systems where the transmitting carrier frequency is locked to the power line frequency utilizing a phase-locked loop, it is often difficult to meet the stringent requirements of such systems. Typically, these requirements are for each unit in the system to generate at least one frequency, typically in the range of 5-30 kHz., which must be kept locked to the powerline frequency, as the latter frequency changes over a range from about 59.9 Hz. to about 60.1 Hz., and over a range of environmental conditions typically including a -40.degree. C. to +85.degree. C. temperature range, with 35 1% power supply voltage variation. The power supply voltage variation may in fact be a step function, when a communications unit transmitter is keyed. Nevertheless, it is required that the communications transmitter maintain an output frequency, and the associated communications receiver recover a transmitted carrier at that frequency, which is within 1 Hz. of the correct frequency over the entire range of environmental conditions. Further, there is frequently a high noise level present on the powerline, so that units at various locations, where the noise characteristics may be different, must maintain the desired frequency and do so with a relationship that tracks the instantaneous frequency of the powerline. Finally, the highly stable frequencies must be generated by apparatus which is of relatively low cost, especially if such apparatus is to be installed at the location of each power user (each residence and business) on the power distribution grid.